1. Field of Invention
The present invention relates to a method for depositing semiconductor films, and more particularly to a chemical vapor deposition (CVD) process utilizing plasma and laser excitation means for high-throughput manufacturing of solar cells devices.
2. Description of Prior Art
Hydrogenated amorphous silicon (a-Si:H) and nano-crystalline silicon (nc-Si:H) are widely used in thin film solar cells because they can be fabricated over large area substrates as required by photovoltaic applications. Compared with amorphous Si, nc-Si:H may produce solar cells with higher efficiency and is more stable against light induced degradation or the Staebler-Wronski effect. Because of its lower absorption coefficient in the visible range of the solar spectrum, however, the nc-Si:H layer in solar cells needs to be 1 to 3 um thick, which is 3 to 10 times thicker than that required of a-Si:H.
Among various methods to form Si thin films over large area substrates, plasma-enhanced chemical vapor deposition (PECVD) which utilizes a capacitively coupled radio frequency (RF) discharge is widely used to form a-Si:H and nc-Si:H in the production of solar cells and thin film transistor (TFT) devices. While a-Si:H based solar cells and TFT devices have been commercially produced by PECVD for years, the production of thicker nc-Si:H films by PECVD is limited by the deposition rate thereof. The film forming rate in the PECVD process may be increased by increasing the RF power input, which increases the number of ionized film forming gas molecules and the energy thereof. As the film forming rate of nc-Si:H is increased by increasing the RF power input, however, the bombardment of the growing nc-Si:H film on the substrate by highly energized ions also increases, thereby generating film structural defects that have deleterious effects on electrical properties thereof. Accordingly, the forming rate of nc-Si:H film by PECVD only reaches approximately 0.5 nm/s in practice (Rosechek et al. Mat. Res. Soc. Symp. Vol 644 (2001)).
To overcome the problems associated with the use of PECVD for forming Si films, a laser-enhanced chemical vapor deposition (CVD) process that utilizes optical energy to decompose the film forming gas has been disclosed, for instance, in Applied Physics Letters, Vol 43, No. 5, pp 454-456. According to this process, the film forming gas resulting from the vapor phase decomposition by photo excitation does not accelerate and bombard the growing film on the substrate. It is therefore possible to form films at high rates with substantially no ion-induced damages at low temperatures.
FIG. 1 is a schematic view of a conventional laser beam CVD apparatus, which comprises a reaction chamber 20; a substrate 22 on which a film is formed; a suceptor 24 incorporating therein a heater for heating the substrate 22; an inlet port 26 for introducing a film forming gas such as silane; an output port 28 for discharging the post-reaction film forming gas; an ultraviolet (UV) laser oscillator 30 disposed outside the reaction chamber 20; an optical system 32 for reducing the diameter of the laser beam emitted from the UV laser oscillator 30; a beam incidence window 34 for transmitting the laser beam which emerged from the optical system 32 into the reaction chamber 20; a laser beam 36 emitted by the laser oscillator 30 for exciting and decomposing the film forming gas; a beam emergence window 38 for transmitting the laser beam 36 out of the reaction chamber 20; and a damper or trap 40 for absorbing the laser beam which has passed through the emergence window 38.
In this apparatus, when the silane gas is introduced from the inlet port 26 into the reaction chamber 20, the silane gas is excited and decomposed by the laser beam 36, which passes in parallel with the substrate 22 along a path spaced apart therefrom by a few millimeters. The reaction product from excitation and decomposition of the silane gas diffuses from the path of the laser beam 36 and deposits over the surface of the substrate 22, thereby forming a silicon film thereon. The post-reaction gas is discharged through the output port 28.
It is possible to form semiconductor films at high rates and low temperatures with the conventional laser beam CVD apparatus described above. However, there are several drawbacks in applying the conventional laser beam CVD process in production of solar cells and TFT devices, which requires dense and uniform semiconductor films deposited over large area substrates.
A problem associated with the conventional laser CVD process described above is that since the reaction product reaches the substrate surface by diffusing away from the laser effected zone, which is defined by the narrow path of the laser beam above the substrate, the concentration of the reaction product on the substrate surface will depend on the distance away from the path of the laser beam, thereby causing the film thickness on the substrate to vary in the direction perpendicular to the path of the laser beam. While it is possible to improve the film uniformity on the substrate by increasing the distance between the beam path and the substrate surface, doing so will adversely decrease the film deposition rate.
Another problem associated with the conventional laser beam CVD process described above is that under high-rate deposition conditions, there is a propensity for the formation of nanoparticles from the gas phase reaction, thereby causing nanoparticles to directly deposit on the substrate surface (for instance, see U.S. Pat. No. 6,248,216 B1). A film composed of aggregates of nanoparticles is inherently porous and has poor adhesion with the substrate compared with a monolithic film formed by condensation of reactant atoms or molecules on the substrate surface, such as films formed by the plasma CVD process. Porosity in semiconductor films can cause oxidation, which would adversely affect the electrical properties thereof, and other reliability problems.
U.S. Pat. No. 4,986,214 issued to Zumoto et al. discloses a laser beam CVD apparatus for forming diamond films. This apparatus includes an ion beam source which irradiates the growing diamond film surface with energetic ions from a non-film forming gas to improve film qualities. While bombardment of growing film by energetic ions may reduce film porosity, it will damage semiconductor films and have deleterious effects on electrical properties thereof as encountered in the PECVD process under high-rate deposition conditions.
Still another problem associated with the conventional laser CVD process described above is the clouding of the window surface inside the reaction chamber because film deposition occurs simultaneously on the surface of the window as the laser beam passes therethrough during the deposition on the substrate. The laser-induced reactions and the film deposition process on the substrate eventually terminate as the reaction product on the window forms an opaque layer so thick that the laser beam cannot effectively pass therethrough.